refactor: drop Xtend build infrastructure from check.core

The module is fully off Xtend: remove the xtend-gen source folder from
build.properties and .classpath, the xtextBuilder/xtextNature from
.project, and the xtend-gen directory marker — matching the other fully
migrated modules. No Require-Bundle change needed (the module declares
no Xtend bundles; xtend2.lib/xbase.lib resolve transitively for the
StringConcatenation-based generators).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: joaodinissf

com.avaloq.tools.ddk.check.core/.classpath

@@ -6,7 +6,6 @@
       <attribute name="ignore_optional_problems" value="true"/>
     </attributes>
   </classpathentry>
-  <classpathentry kind="src" path="xtend-gen">
     <attributes>
       <attribute name="ignore_optional_problems" value="true"/>
     </attributes>

com.avaloq.tools.ddk.check.core/.project

@@ -20,11 +20,6 @@
 			<arguments>
 			</arguments>
 		</buildCommand>
-		<buildCommand>
-			<name>org.eclipse.xtext.ui.shared.xtextBuilder</name>
-			<arguments>
-			</arguments>
-		</buildCommand>
 		<buildCommand>
 			<name>net.sf.eclipsecs.core.CheckstyleBuilder</name>
 			<arguments>
@@ -44,7 +39,6 @@
 	<natures>
 		<nature>org.eclipse.jdt.core.javanature</nature>
 		<nature>org.eclipse.pde.PluginNature</nature>
-		<nature>org.eclipse.xtext.ui.shared.xtextNature</nature>
 		<nature>net.sourceforge.pmd.eclipse.plugin.pmdNature</nature>
 		<nature>net.sf.eclipsecs.core.CheckstyleNature</nature>
 		<nature>edu.umd.cs.findbugs.plugin.eclipse.findbugsNature</nature>

com.avaloq.tools.ddk.check.core/build.properties

@@ -1,6 +1,5 @@
 source.. = src/,\
-           src-gen/,\
-           xtend-gen/
+           src-gen/
 bin.includes = META-INF/,\
                .,\
                plugin.xml,\